1. Field of the Invention
The present invention relates to an electrostatic transfer type image forming apparatus, and particularly to an image forming apparatus such as an electrophotographic printer, a copying machine or the like which uses as a transfer device a transfer roller.
2. Description of the Related Art
Although transfer devices used in electrophotographic printers, copying machines and the like have generally employed a corona transfer process, a roller transfer process has recently been used because it generates little ozone and it has good properties for conveying a recording material, and thus can simplify the carriage means of an image forming apparatus. With regard to fixing devices, these generally comprise a heating roller for use as the fixing device.
However, when an OHP film or dry cardboard (weight 100 g/m.sup.2 or more) is used, a toner image on the recording material is electrostatically offset on a fixing roller on the toner side.
Particularly, in a fixing device where for ease of maintenance there is no cleaning member on the heating roller, a fixing ghost occurs due to the re-transfer of the offset toner to the image surface of the recording material at the period of the fixing roller.
Such an offset phenomenon is prevented by applying to the fixing roller a voltage with the same polarity as that of the toner or by grounding the fixing roller through a low-capacity element such as a diode or the like to produce a self bias.
However, when a bias is applied to the fixing roller in an image forming apparatus which uses a transfer roller for electrostatic transfer, there are the following problems.
When an OHP film or a dry cardboard is used as a recording material, the potential of the recording material after transfer depends upon the type and density of the pattern printed, and thus a potential with a polarity reverse to that of transfer charge is sometimes generated. However, when a voltage with a polarity reverse to that of transfer charge is applied to the fixing roller by an electric power source or a diode in order to prevent offset it is necessary to apply a voltage higher than the potential of the recording material after transfer. As a result, a very high voltage (for example, when a negatively charged toner is used, -2 kV or more) is applied to the fixing roller, and leakage to the temperature detection element of a thermistor, to the pressure roller or the like, each of which contact the fixing roller, easily occurs. This causes errors to be produced in the image forming apparatus, and the need for means for preventing the leakage brings about the complication of the apparatus and an increase in the cost thereof.